I suspect you actually had 2 different amounts of electrolyte.
In chemistry, if you do the exact same thing you will get the exact same result. So your batches were not the same.
I think a lot of problems happen because of measuring the sodium carbonate.
Electronic scales have a feature called auto zero. If you put a weight on the scale and then turn it on, it will automatically read zero. Thats a nice feature.
However, if you turn the scale on, and then add weight a small amount at a time, it will keep re-zeroing, and the result is that the final result is heavier than it should be. The scale reads light.
The solution is to put a known weight on the scale first before adding the chemical. I use an American nickel which is 5 grams. Once you put the weight on the scale, it will defeat the auto-zero function, and it will read 5 grams. Then weigh out your chemical until it weighs 5 grams plus the weight you want. So if you wanted 10 grams of chemical, the scale will read 5 + 10, or 15 grams total.
And don't forget that baked baking soda and commercial washing soda weigh different because of the water in the washing soda.
10.6 grams for baked baking soda (anhydrous sodium carbonate)
12.4 grams for Arm & Hammer washing soda.
With enough distilled water to make 100ml.
If you don't have Arm & Hammer brand, I recommend baking the water out of it first and using the 10.6 gram amount.